Engine exhaust gas may be passed through a catalyst treatment device called a catalytic converter, to reduce emission of undesirable exhaust gas constituents, such as hydrocarbons, carbon monoxide, and oxides of nitrogen. To become catalytically active following a cold start of the engine, catalytic converters rely on heat energy in the exhaust gas passing therethrough. Initiation of catalyst activity occurs at converter light-off temperature, which is typically around 400 degrees Celsius. Following a cold start, the engine may have to operate for over 100 seconds before heat energy transferred from the engine exhaust to the converter elevates the converter temperature to light-off, such that efficient conversion may occur.
To reduce the time to catalyst light-off, exhaust gas heating devices may be provided to introduce additional heat energy in the engine exhaust gas path after a cold start. Heaters have been considered for exhaust gas heating in which an air/fuel mixture is combusted and the combustion heat energy therefrom drawn into the engine exhaust gas path. To minimize the time to catalyst light-off, such heaters must be ignited as soon as possible after a start time, and once ignited, must be operated in a manner wherein maximum heat energy is transferred to the engine exhaust path and ultimately to the converter.
To provide for rapid heater ignition, it has been proposed to apply open loop control to a rate at which air and fuel are admitted to the heater until such time as ignition is detected in the heater. Open loop control of fuel and air are continued after ignition is detected in such an approach, for a period of time until catalytic converter temperature has been increased to a desirable temperature, and maintained at that temperature for a period of time after which it may be assumed that converter temperature may be maintained solely through engine exhaust gas heat energy transfer.
The conditions under which an exhaust gas heating system operates may change significantly. For example, engine exhaust gas backpressure, system temperature and system voltage can change dramatically over the course of even a single heater heating cycle. Furthermore, as the heating system ages, the efficiency of its parts, especially its actuators may deteriorate, and contamination may build up in the system. A significant effect of these changing operating conditions is that an open loop command, such as the open loop fuel and air command of the above-described prior art, may not result in a constant or predictable delivery of fuel or air to the heating system. Under such open loop control, actual air/fuel ratio in the heater may deviate from a beneficial air/fuel ratio, or heat energy generated by the heater may vary from a beneficial energy level. In such cases, the time to reach light-off may be increased, or, in extreme cases of overheating, catalytic converter damage may result.
Accordingly, it would be desirable to close the loop on exhaust gas heater input parameters, such as air or fuel delivered, for application in a control capable of adjusting the quantity of air or fuel commanded to the heater in response thereto. Additionally, it would be desirable to operate a heating system so as to reduce system contamination for increased accuracy over the life of the system.